The present invention relates to a multi-output DC-DC convertor; and, more particularly, to a multi-output DC-DC convertor in which a transformer and other components mounted on the DC-DC convertor are constituted so as to be compact in size and of a thin type so as to operate as a power source of a handy information machine with a built-in DC-DC convertor.
In a conventional DC-DC convertor of the type used in a handy compact size information machine apparatus, such as a notebook type word processor or a personal computer, in general, a switching regulator circuit shown, for example, in Japanese utility model laid-open No. 64988/1992, is in wide use.
In the above-stated conventional machine, at first a direct current input is applied to a circuit in which a transformer and a switching element are connected in series, then the switching element performs an "on" motion or an "off" motion, so that, at a secondary side of the transformer, an alternating current signal having the same frequency as the frequency of the switching element can be obtained. Further, the obtained alternating current is rectified and supplied to a load.
As a method for controlling an output voltage in the above-stated conventional machine, it has been generally known that, in response to the up-and-down motion of the load voltage, the pulse width of the switching element at the primary side of the transformer is varied.
In the handy information processing machine apparatus, it is important for the machine to have a compact size and a thin type power source. However, the trouble with the power source is the size of the transformer used in the power source portion.
However, by using a transformer having no iron core, such as a wire type transformer having a construction in which a primary conductor and a secondary conductor are arranged in parallel, and wherein the primary conductor and the secondary conductor are wound in a plane, an extremely thin transformer can be realized.
However, when it is intended to obtain plural outputs from the above-stated wire-type transformer, it is necessary to couple plural secondary conductors with the primary conductor, to make the electric power transmission most effective. In practice, it is difficult to obtain the above-stated coupling, and further, it causes a problem in that the coupling coefficient between the primary winding and the secondary winding of the transformer decreases.
Further, a transformer having no iron core therein, such as a wire-type transformer, has a longer winding as compared with a transformer having an iron core therein, and also, when the winding ratio between the primary winding and the secondary winding of the transformer becomes large, it causes a problem in that the copper loss particularly at the secondary side of the transformer increases.